There are two known species of cookiecutter sharks: the cookiecutter shark (Isistius brasiliensis) and the largetooth cookiecutter shark (Isistius plutodus). Both are about the same size--reaching lengths of around 50 cm--and both belong to a family of deep water squaloid (Squaliformes--the dogfish sharks) sharks: Dalatiidae (the sleeper sharks belong to this family as well). The two Isistius species are epi- to mesopelagic creatures; it is thought that during the day they remain in the mesopelagic zone (from 200 m to 1000 m below sea level) and then at night they venture nearer to the surface. This behaviour is known as diel (influenced by day-night changes) vertical migration, and many other mesopelagic creatures, from fishes to shrimps, perform this behaviour.
Cookiecutters are also interesting in that they emit a luminous green glow from photophores on their bellies. This is likely used to attract the attention of the cookiecutters' prey: large fishes, pinnipeds and even whales and dolphins, which feed on luminous fishes and squids in the deep sea. When its prey is close enough, the cookiecutter commences its unique attack, by which it got its name. The cookiecutter clamps onto its prey's flesh with its jaws and bites down with its huge, sharp teeth on its lower jaw (the upper jaw teeth are much smaller and finer, but still very sharp, and are used to remain attached to the prey). It is believed that the cookiecutter then twists its body right around, gouging out a plug-like piece of flesh (sometimes the piece may have a thickness of twice the diameter of the mouth). The shark then may create oral suction with its thick fleshy lips, large tongue and strong throat muscles to hoover the piece of flesh out of the prey's body. Tunas, elephant seals, dolphins, whales, swordfish and other large marine animals have all been found with large gouges in their flesh (which do not pose any serious problem to the animals, which are much larger than the cookiecutter shark), and all evidence points to the cookiecutter shark as the culprit, especially since plugs of fish and marine mammal flesh have been recovered from the species' stomach. (Cookiecutters also feed on smaller fishes and squids, in which case the above "cookie-cutting" behaviour is not necessary and the cookiecutter will dispatch the prey like any other shark).
Mouth and teeth of cookiecutter shark (Isistius brasiliensis
All photos reproduced with permission from Australian Museum Online
Lateral view of cookiecutter
shark head.
Body view of
cookiecutter shark
But what of this "giant cookiecutter shark"? Several years ago I attended
an excellent slide lecture show at the Royal Ontario Museum (in my
hometown of Toronto) presented by the renowned Eugenie Clark. It was
entitled "Sea Monsters and Other Mysteries of the Deep" or some such name.
After talking about such interesting things as the New
Zealand 1977 "sea monster" carcass (really a rotten basking shark), weird
abyssal octopuses, and whale sharks, she mentioned briefly a
little story she heard from a colleague who had been working in Alaska.
One night, a dead narwhal was pulled it up along
side the research boat, so that the researchers could examine it in the
morning. It sat in the water all night, and in the morning as the
scientists began to examine it, they found round bites on the animal,
which strongly resembled those left by the cookiecutter shark (see below).
However, these bites were much bigger than the ones usually made by
known cookiecutters, which grow to a maximum of about 50 cm.
If my memory serves me correctly, Dr. Clark toyed, perhaps not too
seriously, with the idea
that there may be larger species of cookicutter sharks inhabiting Arctic
waters. This is a fun idea, but there is an eminently more practical
solution. Richard
Martin, a marine biologist in Vancouver, Canada, wrote to me: